Vacuum Gauge Reading Lower than Normal

gcos · Post by **gcos** » Mon Apr 06, 2015 9:30 pm

Hi All,

I just finished fitting an MJ/E to my 1275 mini and am pleased to say that all is running beautifully.

Except one thing... I'm taking the map sensor feed off of my vacuum gauge feed via a T-Piece (all plumbed into the manifold)... since fitting the megajolt, the vacuum reading on the gauge is much lower than before.

As reference, the MJ gives a load reading of 25KPa at idle. The vacuum gauge used to read ~20 inHg at idle before MJ was fitted but now reads around 15 inHg!

I've done some conversions and in theory the 25kPa load value shown on the MJ is equivalent to around 22.4 inHg on the vacuum gauge. So it seems the engine is still pulling the same vacuum but my vacuum gauge has become confused somehow by the addition of the megajolt map sensor! Surely a vacuum leak in the pipework can't be to blame as this would affect the megajolt load reading too?

Could a restrictive T-Piece be to blame perhaps?

Any thoughts and opinions are welcomed.
Thanks

Alex B · Post by **Alex B** » Wed Apr 08, 2015 3:39 pm

Have you tried switching places on the T-piece?

Alex

gcos · Post by **gcos** » Sun Apr 12, 2015 6:42 pm

Hi Alex,

Thanks for your response. I just tried switching the t-piece connections around and the results were exactly the same as before. I also tried sucking through a pipe attached to the gauge to check the diaphragm and it holds vacuum well so isn't leaking. I also tried setting up the megajolt to hold the idle advance and disconnected the map sensor (blocking the pipe off) and still the vac. gauge reads the same. Finally I tried connecting the vac. gauge on it's own as it was before megajolt with the megajolt holding the idle advance... still reads 15inHg at idle!

I suppose the problem must be with the gauge. This seems odd though as it was working fine before I fitted the megajolt :S

Alex B · Post by **Alex B** » Sun Apr 12, 2015 7:34 pm

I am getting confused. The first conversion programme I picked showed 25 KPa to be 7.38 inHG. 15 inHG is 50.8 KPa, 20 inHG is 67.7 KPa. The first value is quite low for idle imho, the other two quite high.

Alex

gcos · Post by **gcos** » Tue Jun 09, 2015 1:14 pm

Hi Alex,

Sorry for the delay in response!

I think most of the conversion programs are using inHg as absolute pressure whereas on my gauge it is showing depression (i.e. amount that the manifold is below atmospheric pressure). The MAP reading on the MJ on the other hand reads absolute pressure. This means that a lower MAP sensor reading corresponds to a higher Vac gauge reading as the vac. gauge is showing pressure drop not absolute pressure like the MAP sensor.

The chart in this website: http://www.engineeringtoolbox.com/vacuu ... d_460.html
Has conversion for a vac gauge. Notice the difference between the inches mercury absolute and inches mercury gauge columns? It seems most online calculators use inHg absolute whereas my gauge does not. Anyway... using this chart, 26kPa comes out as around 22 inHg (gauge). This is what I think my gauge should be reading!

I have a feeling my gauge has bitten the dust as it still reads incorrectly even when connected as it was originally!

I think my next step will be to swap the gauge into my brothers car (which has a working vac gauge) in order to compare readings.

Thanks for your input, if you have any suggestions based on the above i'd be interested

Post by **brentp** » Mon Oct 19, 2015 6:45 pm

Hi,

Did you ever get this resolved?